1. Field of the Invention
The present invention relates to a laminated wiring board and to its mounting structure. In particular, the invention relates to a laminated wiring board constituted by two kinds of wiring boards having different coefficients of thermal expansion, featuring high mounting reliability and is adapted to transmitting signals at high speeds, and to a mounting structure thereof.
2. Description of the Related Art
An electric device as represented by a semiconductor device using silicon as a chief component includes a number of transistors having very microscopic wiring circuit layers, which are integrated to a high degree, and its size is ever increasing due to an ever increase in the number of transistors. In the electric device, further, attempts have been made to realize fine wiring circuit layers, to decrease the resistance and to decrease the dielectric constant in the interlayer insulating film in order to cope with an increase in the speed of signal processing resulting, therefore, in a decrease in the mechanical strength in the wiring circuit layers and in the interlayer insulating film constituting the electric device.
Accompanying the development in the technology for integrating electric devices in recent years, further, there has been developed a method capable of forming, in the electric device itself, a solid structure and a functional unit having a moving part. For example, there have been put into practical use a fine solid structure called MEMS (micro electro mechanical system) and an electric device having a moving part. In the electric device produced by the above method, however, the solid structure and the moving part are subject to be destroyed due to stress.
When the electric device having such microscopic wiring circuit layers is incorporated in electronic equipment such as a personal computer, a cell phone or a liquid crystal display device, a hierarchical mounting is employed, such as mounting the electric device on a wiring board like a package for containing the electric device (primary mounting) to adjust the size relative to the microscopic wirings and, then, mounting the wiring board on which the electric device is mounted on an external circuit board such as a printed board (secondary mounting).
Japanese Unexamined Patent Publication (Kokai) No. 10-247706 proposes a mounting structure in which, as illustrated in FIG. 4, a wiring board interposed between an electric device 101 and an external circuit board 103 has a two-step structure. Namely, in this mounting structure, a ceramic wiring board 105 is arranged on the side of the electric device 101, and a resin wiring board 107 is arranged on the side of the external circuit board 103.
However, if the above mounting structure is subjected to the cooling/heating cycles in the mounting step such as temperature cycle testing and reflow heating, the connection electrodes 109 provided between the ceramic wiring board 105 and the resin board 107 are subject to be destroyed deteriorating the reliability of mounting. Further, when there is used the electric device 101 having a decreased mechanical strength due to an increase in the size or an increase in the function, application of the above thermal hysteresis causes the electric device 101 itself to be broken.
Namely, the above problems stem from the thermal stress taking place between the ceramic wiring board 105 and the resin board 107 and from the thermal stress taking place between the electric device 101 and the ceramic wiring board 105.